1. Field of the Invention
The present invention relates to an imaging lens that focuses an optical image of an object on an imaging device, such as a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor), a camera module that converts the optical image formed by the imaging lens into an image signal, and an imaging apparatus, such as a digital still camera, a mobile phone with a camera, or a personal digital assistant (PDA), that is provided with the imaging lens and captures images.
2. Description of the Related Art
In recent years, as personal computers have spread to the home, digital still cameras capable of inputting image information, such as a landscape or a portrait image, to personal computers have spread rapidly. In addition, in many cases, camera modules for inputting images are mounted to mobile phones. An image device, such as a CCD or a CMOS, has been used as the device having an imaging function. In recent years, with a reduction in the size of the imaging device, there is demand for a small imaging apparatus and a small imaging lens provided in the imaging apparatus. At the same time, the number of pixels of the imaging device has been increased, and there is demand for an imaging lens having high resolution and high performance.
JP-A-2002-221659, JP-A-2004-302057, JP-A-2004-341013, JP-A-2005-24581, JP-A-2005-24889, Japanese Patent No. 3424030, JP-A-2007-17984, and JP-A-2007-219079 disclose imaging lenses having a three-lens structure or a four-lens structure. As described in JP-A-2002-221659, JP-A-2004-302057, JP-A-2004-341013, JP-A-2005-24581, JP-A-2005-24889, Japanese Patent No. 3424030, JP-A-2007-17984, and JP-A-2007-219079, particularly, the following imaging lenses each having a four-lens structure have been known: an imaging lens in which four lenses having positive, negative, positive, and positive powers are arranged in this order from the object side; and an imaging lens in which four lenses having positive, negative, positive, and negative powers are arranged in this order from the object side. In the imaging lens having the four-lens structure, in many cases, the lens closest to the image side includes an object-side surface that is convex near the optical axis (in the vicinity of the optical axis). In Examples 5 and 9 disclosed in JP-A-2007-17984, four lenses having positive, negative, positive, and negative powers are arranged, and the lens closest to the image side includes an object-side surface that is concave near the optical axis.
As described above, in recent years, the size of the imaging device has been reduced, and the number of pixels thereof has increased. In the past, particularly, there was demand for an imaging lens used in a portable camera module having a low cost and small size. However, in recent years, the number of pixels of an imaging device of a portable camera module has increased, and there is a demand for an imaging device having high performance. Therefore, it is desirable to develop a variety of lenses considering all factors, such as cost, performance, and size. It is desirable to develop an imaging lens that has a low cost and high performance with performance suitable for mounting to a digital camera. The above-mentioned lenses are insufficient in obtaining both high imaging performance and reduce the size. In addition, although JP-A-2007-17984 discloses various types of imaging lenses having a four-lens structure, optimization conditions for each structural example are not sufficiently examined.